This invention relates to apparatus for maintaining a low-temperature environment, and, more particularly, to an improved construction for a vacuum package dewar.
Many types of electronic devices operate most efficiently at reduced temperatures. For example, infrared sensors using focal plane array, charge-coupled device must be operated at a temperature of 77K. or less. Cooling of the sensor elements is accomplished by placing the focal plane array into an evacuated enclosure having an infrared-transmissive window. The sensor within the enclosure, termed a vacuum package or dewar, is cooled to the required reduced temperature by cryogenic gas cooling, Joule-Thomson cooling, thermoelectric cooling, or other operable technique. The vacuum enclosure prevents heat from the surroundings from being conducted to the sensor to warm it at a rate faster than heat can be removed by the cooling apparatus. The present invention relates to the construction of the dewar and the method of its fabrication.
In the conventional practice, the dewar is formed of a two-part housing. An upper vacuum housing includes the infrared-transmissive window, and a lower vacuum housing contains the focal plane array sensor, its support that provides the conductive heat path to the cooling apparatus, and related structure. During assembly of the instrument, the support structure and sensor are installed within the lower vacuum housing, and electrical connections are made to feedthroughs built into the wall of the lower vacuum housing. The sensor is installed in the lower vacuum housing, and the upper vacuum housing is fixed to a flange on the lower vacuum housing. The interior of the device is evacuated and sealed off to complete the fabrication process.
The present invention relates to the manner of construct ion of the lower vacuum housing, prior to the above-described assembly steps, and to the resulting dewar. The lower vacuum housing is usually prepared by first building a number of subassemblies and joining the subassemblies together. Thus, typically, an outer housing, a coldfinger tube, an adaptor collar, a platform, a ceramic insulator, electrical feedthrough leads, a flange, and a header are fabricated. The coldfinger tube and the adaptor collar are welded or soldered together. A tip-off tube is welded to the flange. The feedthrough leads are brazed to the ceramic insulator. The ceramic insulator is brazed to an opening in the outer housing. The header and flange are welded to the outer housing. The coldfinger tube/adaptor collar is welded to the outer housing. All of these assembly steps require separate joining operations, separate inspections and tests, and the tracking of the various subassemblies through the joining operations.
This approach, while effective in producing an operable lower vacuum housing suitable to receive the focal plane array of the sensor, has some drawbacks. First, the many subassembly and assembly steps are time consuming and costly. Welding procedures in particular require extensive setup and care in positioning the parts, even when automated procedures are used. Second, there may be reliability problems resulting from the many types of joints and procedures utilized. Third, these joining operations and the order in which they are performed dictate the material types which can be used for the various components, which sometimes leads to less than optimal choices for the materials.
There is a need for an improved approach to the fabrication of such dewar structures, which reduces the cost and improves the reliability of the final structure, and permits wider flexibility in the selection in materials used in the components. The present invention fulfills this need, and further provides related advantages.